Microcomputers are sophisticated, general purpose logic devices which can be programmed to perform a wide variety of useful control functions in industrial and communications equipment, large-scale and medium-scale computer peripheral and terminal hardware, automobiles and other transportation media, amusement and education devices and the like. Generally, an entire spectrum of microcomputers is presently available in the commercial marketplace. The MC6801 microcomputer commercially available from Motorola, Inc., is an 8-bit microcomputer comprising a CPU, 128 bytes of random access memory (RAM), 2K bytes of EPROM, a 16-bit timer, and four I/O ports for communicating with external equipment.
It is known in the microcomputer technology to provide an EPROM as a component part of a overall single-chip microcomputer. For example, the MCS8748 commercially available from Intel Corporation is a single-chip microcomputer which includes an EPROM. The MCS8748 EPROM is programmed utilizing a microcomputer development system. The microcomputer development system requires an independent processor, operating under the control of an EPROM loading program, to program the EPROM. The MCS8748 contains at least three pins, out of a total of 40 pins, whose function is dedicated to pogramming the EPROM. A T0 pin is set low to select the programming mode; an EA pin is raised to 25 volts to activate the programming mode; and a PROG pin is raised to 25 volts for a 50 millisecond pulse to program data into the EPROM.
It is desirable to eliminate the necessity for dedicated programming pins in a microcomputer having an EPROM, since such programming pins are used only during the EPROM programming operation and might otherwise be more usefully employed in providing other I/O or control functions to the microcomputer.